Finite element modelling for evaluation of compression/release stabilized transfemoral prosthetic socket

Abstract

Amputees who lost their lower limbs are mostly fitted with prostheses to facilitate standing and locomotion. Prosthetic socket, which is the direct contact and interaction of the prosthesis with the residual limb as the crucial element that influences the quality of prosthesis fitting. Transfemoral amputees complain pain, abrasion, humidity, inconvenient donning and doffing, and poor control of prosthesis with conventional socket designs. The newly developed compression/release stabilized (CRS) concept has the aim to increase the prosthetic socket control efficiency and stability through pre-compressing the residual limb surface along the shaft of the bone. In transfemoral CRS prosthetic socket design, four longitudinal depressions pre-compress on soft tissue along the femoral bone to stabilize the residual limb and four release areas adjacent to the compressed parts allow tissues to displace. The Further motion of the bone within the soft tissues is minimized with this pre-compression. The momentum directly transfers through the compressed thigh muscle to the socket without soft tissues motion. There is no need to have a proximal socket brim because the residual limb is better controlled, and more weight is supported along the femoral shaft. However, there are little publications and investigation on the transfemoral CRS prosthetic socket design. This study aimed to better understand the stress and strain distribution of the transfemoral residual limb inside transfemoral CRS socket by finite element method. A pilot study was conducted to fit a trial prosthetic socket on a healthy subject and a transfemoral amputee to find an appropriate reference for the geometry of the compression areas of the CRS socket design. An FE model of the transfemoral residual limb was established from the MR image of the transfemoral subject. The CRS socket design was constructed by using the 3D reconstructed geometry of the residual limb to increase the accuracy of the assembled FE models. Two stages were performed in this study: first, the CRS socket model underwent 106mm displacement (which was vertical displacement of the residual limb from just in touch with socket to the fully donned position) to simulate the donning procedure; second, an 800N force was applied as the subject's total body weight on the femoral head to exam the loading effect. The baseline of the compression value was set as 15mm derived from the pilot study on the evaluation of different compression depth. For comparison, a CRS socket with 10mm compression was constructed as well. The FE results were validated experimentally with interface pressure measurement. The effects of material with a different coefficient of friction (0.3, 0.4 and 0.5) and the inclusion of silicone liner were simulated by the FE model. The results showed that the maximum FE predicted normal pressure was 212 kPa in the proximal anterior-medial side of residual limb surface. It was higher than the recent literature on the transfemoral study which indicated that the further compression in CRS socket design achieved. When the simulated coefficient of friction was reduced from 0.5 to 0.4 and then 0.4 to 0.3, the maximum contact pressure was reduced 26.73% and 2.56% respectively. When the depth of the compression areas was decreased from 15mm to 10mm, the maximum contact pressure in the donning and loading conditions were reduced by 14.68% and 7.78% respectively. With a 3mm thick silicone liner, there was much about 40% maximum pressure reduction. The patient responded positively with the CRS socket design in this study. This method may help the prosthetist to design the CRS socket more accurate and faster.